LLD Interview field guide · 2026

Low-level design.One decision at a time.

Learn to turn requirements into objects, contracts, and trade-offs. Follow one focused path from first principles to complete interview systems, with every lesson designed for clear recall under pressure.

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55
articles
12
chapters
15+
design patterns
12
system drills

The interview loop

A design is a sequence of decisions, not a class diagram.

  1. 01Clarify

    Turn ambiguity into constraints.

  2. 02Model

    Assign state and responsibility.

  3. 03Connect

    Choose stable contracts.

  4. 04Stress

    Test change, failure, and concurrency.

Course map · sequential by design

From first principles to full systems.

Read in order for a guided path, or jump directly to the pattern or interview problem you need today.

01

Model the problem

Foundations of LLD

2

Build the vocabulary, object-oriented instincts, and interview workflow that every later design depends on.

  1. 01Introduction to Low-Level Design
  2. 02Software Design Principles
02

Control change

SOLID Principles

5

Use five practical design constraints to keep classes focused, replaceable, and easy to extend.

  1. 01Single Responsibility Principle (SRP)
  2. 02Open–Closed Principle (OCP)
  3. 03Liskov Substitution Principle (LSP)
  4. 04Interface Segregation Principle (ISP)
  5. 05Dependency Inversion Principle (DIP)
03

Create deliberately

Creational Patterns

6

Separate construction from use with factories, builders, prototypes, and carefully scoped object lifecycles.

  1. 01Introduction To Design Patterns
  2. 02Singleton Pattern
  3. 03Factory Method
  4. 04Builder Pattern
  5. 05Abstract Factory
  6. 06Prototype Pattern
04

Compose the system

Structural Patterns

7

Shape object relationships with adapters, decorators, facades, proxies, bridges, composites, and flyweights.

  1. 01Adapter Pattern
  2. 02Decorator Pattern
  3. 03Facade Pattern
  4. 04Composite Pattern
  5. 05Proxy Pattern
  6. 06Bridge Pattern
  7. 07Flyweight Pattern
05

Coordinate behaviour

Behavioural Patterns

10

Design communication, state, commands, iteration, and algorithm selection without tight coupling.

  1. 01Iterator Pattern
  2. 02Observer Pattern
  3. 03Strategy Pattern
  4. 04Command Pattern
  5. 05Template Method
  6. 06State Pattern
  7. 07Chain Of Responsibility
  8. 08Visitor Pattern
  9. 09Mediator Pattern
  10. 10Memento Pattern
06

Protect shared state

Concurrency

7

Reason about threads, executors, synchronization, deadlocks, locks, and producer-consumer coordination.

  1. 01Multithreading And Concurrency
  2. 02Creating And Managing Threads
  3. 03Thread Pools And Executors
  4. 04Thread Safety And Synchronization
  5. 05Locks And Synchronization Mechanism
  6. 06Deadlock And Prevention Techniques
  7. 07Producer Consumer Problem
07

Invert dependencies

Dependency Injection

1

Move object wiring outside business logic so implementations remain replaceable and testable.

  1. 01Dependency Injection
08

Design failure paths

Resilience & Errors

2

Model exceptions, recovery, retries, and degradation as intentional parts of the design.

  1. 01Exception Handling in LLD
  2. 02Building Resilient Systems
09

Connect the layers

LLD Best Practices

3

Bring APIs, persistence, and a repeatable interview approach into one coherent design process.

  1. 01All About APIs
  2. 02Database Design And Integration
  3. 03How to Approach an LLD Interview
10

Apply the toolkit

Interview Systems I

5

Design physical and workflow-heavy systems: parking, logging, traffic signals, vending, and tasks.

  1. 01Parking Lot
  2. 02Logging Framework
  3. 03Traffic Signal System
  4. 04Vending Machine
  5. 05Task Management System
11

Model transactions

Interview Systems II

3

Practice event delivery, financial workflows, and reservation-oriented domain models.

  1. 01Pub/Sub System
  2. 02ATM Machine
  3. 03Hotel Management System
12

Handle evolving state

Interview Systems III

4

Finish with elevators, wallets, ride booking, and streaming platforms built for change.

  1. 01Elevator System
  2. 02Digital Wallet
  3. 03Ride Booking App
  4. 04Music Streaming Platform

One interview at a time

Start with the object in front of you.

Requirements first. Patterns only when they earn their place.

Begin chapter 01